Posted: Sat, Jun 7, 2014, 9:36 AM ET (1336 GMT)
An analysis of the chemistry of lunar meteorites and rocks has provided scientists with new evidence that the Moon formed as the result of a collision of a protoplanet with the early Earth. Isotopic analysis of oxygen found in several lunar meteorites, as well as rocks returned from the Moon by the Apollo missions, showed that the lunar samples had a slight increase in oxygen-17 concentrations versus terrestrial rocks. That enhanced oxygen-17, scientists argue, is likely from rocks from the Mars-sized planetesimal that collided with the Earth, the leading model to form the Moon. Previous tests had shown little isotopic variation between terrestrial and lunar rocks, even though impact formation models concluded more debris from the colliding object should have ended up on the Moon.

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